The invention relates to a storage system having flash disks as storage media for storing data, and also to a write distribution method.
JP-A-5-27924 discloses a storage system having flash disks as storage media for storing data. A flash disk is a semiconductor disk device that uses nonvolatile flash memory. Flash memory is composed of blocks, and each of these blocks is composed of pages. A page is a read/write unit and is the same size, 512 bytes, as a sector, which is the minimum read/write unit for a hard disk device such as a magnetic disk device. The general method for rewriting data within a page is to erase old data and then write new data. A block is an erasion unit (generally 16 Kbytes). Even if only pages, which are part of a block, are to be erased, it is still necessary erase the entire block
However, a flash disk that uses flash memory having the above-described characteristics has the following limitations: bit errors occur at a certain rate in data stored in flash memory; while data is erased by blocks, the life of each block in terms of the number of times each can be erased, allowing only between 100,000 and 1,000,000 erasions, is short.
It is an object of the invention to balance the write-count in flash memory and extend the useful life of the flash memory. “Balancing” means making the values as close to each other as possible so that no one unit is used excessively when compared to the others.